Watermarks on paper attached to substrates through which light cannot pass can be observed by converting a high resolution digital image of the region into a suite of colour spaces. An image is comprised of a variety of layers or textures which can be separated. This allows pixels of interest to be isolated which may include faded writing, obscured text or watermarks. A watermark of a post horn surrounded by a shield was discovered on the inner paste down on the lower board of the St Cuthbert Gospel (British Library Additional MS. 89000) using colour space analysis on a digital image. This technique was applied using ImageJ, an open source image processing software (downloadable at http://rsb.info.nih.gov/ij/) which forms the engine for a free-to-use website called ‘retroReveal’, designed by Hal Erickson and accessible at http://www.retroreveal.org. Images can be uploaded onto retroReveal and converted into different colour spaces which are then instantly available for viewing and download. Further image processing with ImageJ can enhance the observed watermarks. This was an entirely non-destructive, non-analytic imaging technique which required only a high resolution digital image. Digitisation projects generate large amounts of high resolution images which can be manipulated to discover hidden information without the need to access the item. This has significant implications for the long-term study and preservation of cultural heritage collection items.
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